Not applicable.
Not applicable.
The present invention relates to air compressors, and in particular to a system for cooling air compressors.
Many types and constructions of air compressors are known in the art. While some use rotating impellers to pressurize the air, positive displacement compressors are most common. Positive displacement compressors include a piston, crankshaft, connecting rod, cylinder and valve head. Typical compressors have one or two cylinders and a corresponding number of pistons. Two cylinder compressors operate the same as single cylinder compressors, however, each revolution of the crankshaft causes two compression strokes, one for each piston.
The crankshaft is ordinarily powered by an electric motor or a gas engine. At the top of the cylinder, there is a valve head having inlet and discharge valves controlling the passage of air into and out of the cylinder. As the crankshaft is rotated, the connecting rod moves the piston up and down within the cylinder. As the piston moves down, a vacuum is created which draws outside air past the inlet valve and into the cylinder. As the piston moves up, the air in the cylinder is compressed which shuts the inlet valve and opens the discharge valve. Compression of the air also generates considerable heat.
Many positive displacement compressors are designed with an oil bath that splashes the connecting rod bearings and cylinder walls. For such compressors, one or more annular compression rings on the pistons seal against the inner diameter of the cylinders so that the lubricating oil does not mix with the compressed air. Often, however, the compression rings are not completely effective to prevent the oil from entering in the compressed air in aerosol form, which is intolerable in some applications. Also, oil lubricated compressors require maintenance and replacement of the oil as well as that the compressor be operated on a generally level surface.
Oilless compressors provide a solution to these problems. Typically, such compressors use sealed connecting rod bearings and compression rings made of a self-lubricating material, such as PTFE. However, because there is no oil to lubricate the moving parts, the temperature within the crankcase and cylinders is higher. And, since self-lubricating materials, like most materials, degrade over time in high temperature environments, the useful life of the compression rings is directly related to the effectiveness of the cooling system of the compressor.
Some compressors have an open crankcase allowing outside air to pass therethrough to cool the cylinders and compression rings. However, compressors with open crankcases are often noisy and can require additional maintenance due to dust and debris entering the crankcase and damaging the connecting rod bearings, compression rings and/or cylinder walls. As such, it is desirable to completely enclose the crankcase. Compressors with enclosed crankcases use blower wheels operated by the drive motor to direct air past the exterior of the crankcase and cylinders.
Often, however, such a design does not adequately cool the cylinders and compression rings. This is because the cylinders are ordinarily made of cast iron. Cast iron provides a hard, smooth inner bore creating a low-friction bearing surface for the compression ring, and the casting process provides a cost effective means of forming cooling fins around the cylinder. However, cast iron has a relatively low thermal conductively, roughly half that of aluminum.
One solution is disclosed in U.S. Pat. No. 4,492,533 issued to Tsuge on Jan. 8, 1985. Here, the air compressor has its drive unit, crankcase and cylinders confined within a sound-proof box. The compressor includes a fan and the box has air inlet and outlet openings. The crankcase also has a plurality of bores defining passages for air to travel to cool the connector rod bearings and piston rings. While this design solves some of the aforementioned problems, it requires a sound-proof box, which is not totally enclosed so that the debris can enter and increase friction between moving parts.
Accordingly, a need exists in the art for an improved cooling system for an oilless air compressor with a completely enclosed crankcase.
The present invention provides an oilless air compressor having a cooling system with a drive unit operating a crankshaft disposed within a crankcase and to which is attached a piston movable within a compression cylinder. The compression cylinder includes a thermally conductive aluminum alloy cylinder insert having a bore in which the corresponding piston rides. A thermally conductive aluminum alloy heat sink structure is cast integrally to the outer diameter of each cylinder insert and the cylinder is connected to the crankcase such that the compression cylinder and the crankcase are completely enclosed.
The cylinder insert is preferably made of a low silicon, high melting point aluminum alloy, preferably having a silicon content of less than one percent and a melting point of more than 600 degrees Celsius.
In one preferred form, the present invention includes an oilless air compressor having a positive displacement compressor unit with a pair of reciprocating pistons movable within a pair of offset compression cylinders forming a V-configuration. Each compression cylinder includes a thermally conductive and low silicon aluminum alloy cylinder insert and a thermally conductive aluminum alloy heat sink structure cast integrally to the outer diameter of the cylinder insert. The heat sink includes a plurality of annular cooling fins. A drive unit operates a crankshaft within a completely enclosed crankcase to reciprocate the pistons within the compression cylinders. A blower wheel external to the crankcase is rotated by the drive unit to direct air past the heat sink to cool the internal components of the compressor.
Thus, the present invention provides an air compressor in which the crankcase can be completely enclosed without requiring cooling air to pass therethrough. This allows the air compressor to operate quieter than open crankcase compressors and prevents premature wear of piston seals, cylinders and crankshaft bearings. Even though the crankcase is completely enclosed, it is cooled sufficiently by blowing external air past the outside of uniquely constructed compression cylinders having a cylinder insert and a heat sink, both made of an aluminum alloy having a high thermal conductivity. Moreover, the aluminum alloy has a high melting point so that the heat sink can be cast about the cylinder insert without losing structural integrity during the casting process. Still further, the aluminum alloy has a low silicon content so that the inner diameter of the cylinder insert can be machined to a smooth finish after the casting process and then anodized to a suitable hardness, without degradation of the surface finish. As such, only one machining operation is required, which lowers cost.
The foregoing and other objects and advantages of the invention will appear from the following description. In this description reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention.